A typical communications antenna consists of a number of radiating elements, a feeding network and a reflector. The purpose of the feeding network is to distribute a signal from a single connector to all radiating elements. The feeding network usually consists of controlled impedance transmission lines. The antennas need to be impedance matched to a predefined value, usually 50 ohm or 75 ohm, otherwise power fed into the antenna will be reflected back to its source instead of being radiated by the radiating elements, with poor efficiency as a result.
The signal needs to be split between the radiating elements in a transmission case, and combined from the radiating elements in a reception case, see FIG. 1. This is usually done using the same network, which is reciprocal. If the splitters/combiners consist of just one junction between 50 lines, impedance match would not be maintained, and the common port would be 25 ohm instead of 50 ohm. Therefore the splitter/combiner usually also provides an impedance transformation circuit that gives 50 ohm impedance at all three ports.
The antennas comprise coaxial lines that are parallel to a reflector, and that have connectors placed usually at an antenna bottom, with the connectors pointing in a direction parallel to the reflector. The connectors are usually attached to a bottom plate that is perpendicular to the reflector. A centre conductor is connected to a centre pin in the coaxial connector at the antenna bottom plate. This connector is used to connect a feeder.
To obtain cellular coverage at higher frequencies, antennas with higher gain without reducing the aperture excessively are required. Such antennas can be realized using large coaxial lines with air as dielectric.
Some manufacturers use coaxial lines with square cross-section tubes, as an outer conductor, together with a circular central conductor, as an inner conductor, see FIG. 2. The impedance of the line depends on the ratio between the outer conductor and the inner conductor, and what type of dielectric material that is used.
The inner conductor is suspended in square tubes using small pieces of dielectric support means for example made of polytetrafluoroethylene (PTFE). These dielectric support means are made as small as possible in order to maintain the line impedance. The necessary impedance transformation is obtained by machining the centre conductor or by other means such as increasing the size of the dielectric supports and optimizing their position.
Also losses within the antenna must be kept to a minimum in order to obtain a high system receiving sensitivity, and transmitting efficiency. Losses in the antenna are mainly due to impedance mismatch or losses in the antenna feeding network.
Antennas are sensitive to different kinds of disturbances, as described above. Another common disturbance that has to be avoided is intermodulation in the antenna. Antennas comprise different parts where all of them have to be intermodulation-free parts.
One problem is to connect the centre conductor of the coaxial line to the antenna connector. The connector that is used to connect a feeder cable to the antenna-feeding network is usually placed at the bottom of the antenna, and is usually attached to the bottom plate that is perpendicular to the coaxial lines that are inside the antenna. The centre pin is located in the connector, which is to be connected to the centre conductor in the coaxial line of the final line of the antenna. The outer signal path of the coaxial connector is typically connected to the end bottom plate made of a conducting material such as metal. The outer current then has to flow through the end bottom plate to the outer conductor of the feeding circuit coaxial lines. There are two requirements that must be fulfilled for the connection between the end bottom plate and both the coaxial connector and the antenna feeder outer conductor. One is that impedance matching must be maintained, and the second is that a junction between the end bottom plate and the reflector must not generate intermodulation when the antenna is subject to high power.
Both these requirements demand a consistent electrical connection between the end bottom plate and the reflector. Even if a correct impedance match is obtained, a bad electrical connection can generate intermodulation.
A further problem is that if the connector uses the centre pin to connect the centre conductor as described above, due to mechanical constraints, no standard connector is usually available, and hence a custom-made item must be used. Such non-standard connectors are much more expensive than standard connectors, and have longer lead times than standard ones.
One solution to bad electrical connection is to braze the end bottom plate to the reflector. The use of an electrically conductive bottom plate as support for the connector, and which also is used as coaxial outer conductor, introduces two electrical interfaces that potentially can generate intermodulation. One interface is between the connector and the bottom plate, and the second interface is between the bottom plate and the antenna coaxial line outer conductor. The disadvantage of this solution is that it is a very costly process, and that it is difficult to maintain a consistent manufacturing quality that would ensure low or no intermodulation. This does not either solve the problem of the connection between the connector and the bottom plate. This connection can also be subject to mechanical stress, which increases the risk for intermodulation.
Most antennas today use coaxial cables with a polymer dielectric such as PTFE and the problems above are avoided. However, the problem with this solution is that the lines introduce significant losses, this reducing the gain of the antenna.